Skin and wound dressings are designed to undertake a number of important functions to aid the process of healing. Experts agree on most of the functions that an ideal dressing should provide, and these include:                Donation of moisture to dry wounds        Absorption of excess fluid from weeping wounds        Maintenance of a moist environment around the wound bed        Binding of water sufficiently well to prevent maceration (water-logging) of the normal tissue        Aiding debridement (removal of dead tissue and scar material)        Prevention of infection and provision of a barrier to escaping or invading microbes        Killing infecting microbes        Cushioning against further physical trauma        Maintaining an optimum temperature through thermal insulation        Allowing ingress of plentiful oxygen        Soothing painful and inflamed open wound sites        Flexibly adapting to the shape of the wound site        Keeping its physical integrity so that fragmented dressing debris is not left in the wound        Exerting no cytotoxic nor physically damaging effects on the healing cells.        
In addition, the handling and physical design characteristics should make the dressing easy to use and comfortable to wear. For storage and distribution purposes, the dressing should be stable at ambient temperatures, and robust. Ideally it should be simple to manufacture, in order to allow its production and sale at a price that is affordable for widespread use.
These and other demands make the design of an ideal wound dressing almost impossible. To date, all wound dressings are a compromise, such that none offers all of the much desired characteristics in one product. For this reason, there are numerous different wound dressings on the market, and the typical nurse caring for patients with wounds needing professional care will select different dressings for different wounds and for wounds at different phases of the wound healing process. Manufacturers are constantly seeking new ways to make more effective wound dressings, which means that they are trying to make dressings that incorporate more of the characteristics and functions listed above. With the achievement of each new benefit, the cause of improved patient welfare is advanced, as the result of faster healing, reduction of pain and improvement in the quality of life. Medical care in general can benefit from such progress. Although these advanced, “active” dressings usually cost more, they can reduce the overall time during which a wound needs attention and reduce the amount of nursing time devoted to frequent changes of dressing. This drives down the huge cost borne by modern society in caring for wounds.
The invention described here is concerned with improving the performance of wound dressings, in terms of the features listed above.
When considering this list of requirements, it soon becomes clear that many of the demands seem to be contradictory. For example, a dressing that donates moisture would not, at first sight, be expected to be able to absorb water—the two functions seem to be in opposition to each other. Another example is the need simultaneously to provide a cushioning effect and an efficient inflow of oxygen, whilst preventing dryness. It would be expected that a dressing bulky enough to act as a cushion or shock absorber would inevitably provide a barrier to oxygen ingress, especially if the whole of the surface is sealed to keep moisture in. For this reason, some wound dressings are compound structures, made up from different layers, each with a different function and role. In fact, practitioners often mix and match different dressings from different manufacturers to produce their own compound structures, with highly variable results. Compound dressings need to be designed to work as an integrated whole, or the components may interact with each other to inhibit or neutralise the effects designed to operate on the wound.
Wounds frequently become infected. Wound dressings may carry antiseptic substances, and the physical protection they provide prevents ingress of extra infecting microbes, although this microbial exclusion is seldom absolute. Antiseptic substances carried on the dressing pad are not usually very effective, possibly because they do not readily diffuse into the wound at a steady rate. Moreover, the most effective substances, antibiotics, are not available for routine use, because of the ever-present problems of emerging drug resistance.
Hydrogen peroxide (H2O2) is a known antimicrobial substance with many advantages. It is produced naturally in the body by white blood cells as part of the immune defense activities in response to infection. There are no known microbial evasion mechanisms by which microbes can escape its effects and it has a short lifetime, very rapidly breaking down to water and oxygen in the tissues. It therefore does not accumulate to dangerous levels. When it is to be applied topically (e.g. to treat acne), its effectiveness is enhanced by the fact that it readily penetrates the skin surface to reach underlying sites of infection.
As hydrogen peroxide is so beneficial, it has been used for many years as an anti-microbial substance for cleansing wounds of all kinds and as a biologically compatible general antiseptic. In particular, hydrogen peroxide-containing ointments have been used, e.g., for treatment of leg ulcers, pressure sores, minor wounds and infection. There are, however, problems associated with the use of hydrogen peroxide. Hydrogen peroxide solution is very unstable and is readily oxidised to water and oxygen; further, hydrogen peroxide at high concentration can be damaging to normal skin and to cells responsible for healing in the wound bed. It is very difficult or even impossible to use hydrogen peroxide as part of a pre-dosed wound dressing: its instability would make for a product with a relatively short shelf-life, and dosing at the point of application would still not provide a sustained delivery over a usefully prolonged period. When it is used in wound treatment (as described in the British Pharmacopoeia, for example) high concentrations (typically 3%) are needed to achieve a powerful antimicrobial effect over a very short time interval. Even this type of short burst can be effective, because of the great effectiveness of hydrogen peroxide, but there is the further disadvantage that such high concentrations can be relatively damaging to host cells and can impede the healing process. For this reason, use of hydrogen peroxide tends to be restricted to initial clean-up and sterilisation of wounds. Even so, it is a natural defense substance, produced by the body's own cells (at lower concentrations) and it is increasingly recognised as an intercellular and intracellular messenger molecule, involved in cell to cell molecular signalling and regulation. Undoubtedly, hydrogen peroxide is potentially a very beneficial multifaceted healing agent, if it can be used at the right concentrations and in the appropriate time course.
U.S. Pat. No. 4,576,817 proposes a bacteriostatic fibrous wound dressing incorporating dry enzymes such as glucose oxidase and lactoperoxidase to generate e.g. hydrogen peroxide and hypoiodite on contact with serum.
WO 01/28600 discloses a wound dressing including dry glucose oxidase, dry lactoperoxidase and an iodide salt in a polymeric matrix. The glucose oxidase catalyses an oxidation reaction of glucose present in body fluids of a wound site to generate hydrogen peroxide. The action of lactoperoxidase on hydrogen peroxide and iodide generates elemental iodine, which is a powerful anti-infective agent.
The wound dressings disclosed in U.S. Pat. No. 4,576,817 and WO 01/28600 rely on use of water in body fluids for hydrating dried enzyme. This inevitably leads to a delay between application of such a dressing to a wound and functioning of enzyme-based reactions.